Prior to being subjected to thermal diffusion/penetration treatment, coating treatment to form hard ceramic coatings, plating treatment or the like thermal surface treatment, metallic works made of steel, aluminum, titanium or nickel, for instance, are generally subjected to various types of pretreatment, for example cleaning, degreasing, acid pickling and treatment with a molten flux. Thus, for example, alkali degreasing and/or cleaning with an organic solvent is selectively applied to carbon steel works before such thermal treatment as carburization or nitriding. For nitriding or the like thermal treatment of stainless steel works, a step of removing surface oxidized layers by washing with a hydrofluoric acid-nitric acid mixture is added to the above-mentioned pretreatment step or steps. In the case of such thermal treatment as physical vapor deposition (PVD) or chemical vapor deposition (CVD) for forming hard ceramic coating layers, such intermediate processing as nickel plating is conducted as a pretreatment step in some instances for improving the adhesion of coating layers to substrate metallic works. For such thermal treatment as plating treatment in a molten zinc or aluminum bath, substrate works are pretreated with a molten flux following degreasing and acid pickling to thereby realize an increased surface activity, or substrate works are maintained at a temperature above the contemplated thermal treatment temperature for a certain period of time and then gaseous hydrogen or a gas containing a high concentration of hydrogen is introduced into the system for reducing the substrate work surface in the resulting reducing atmosphere to achieve the same purpose. The primary object of these pretreatment processes is to activate the surface of substrate metallic works to thereby facilitate the thermal treatment proper and produce maximum treatment effects. However, recent regulations against waste water discharge, regulations against the use of fluorocarbon species, aggravated working conditions and other factors have made it difficult to continue the commercial use of most of the above-mentioned pretreatment processes and have caused increases in pretreatment cost year by year. Furthermore, the pretreatment process comprising maintaining substrate steel works in a reducing gas atmosphere at an elevated temperature prior to plating treatment using molten zinc or aluminum not only requires an expensive reducing gas in large quantities but also involves the problem that the efficiency of plating is impaired by selective oxidation of valuable elements contained in steel materials, for example Mn, Si and Al. It is not easy to maintain such elements in a completely reduced state in the temperature range not higher than 780.degree. C. as compared with Fe, Zn and the like; such elements are susceptible to oxidation and are readily oxidized in the temperature range of about 500.degree.-600.degree. C. As a result, there arises the above-mentioned problem, namely the plating efficiency decreases due to oxidation.
As mentioned hereinabove, the prior art pretreatment processes to be applied to substrate metallic works before the subsequent thermal treatment proper still encounter such problems as increases in pretreatment cost, environmental pollution problems and deterioration of performance characteristics of metallic materials themselves. Solution of these problems is earnestly desired.